The present invention pertains to thermoelectric materials, and more particularly to a high performance thermoelectric material and method of fabrication.
Thermoelectric power is generated by the Seebeck effect in the thermoelectric material that is used in typical thermoelectric devices. Most commonly, thermoelectric devices are constructed of an N-type and a P-type semiconductor material, such as bismuth telluride. The N-type and P-type semiconductor material are electrically connected in series and thermally connected in parallel. When heat is passed through the material, electricity is generated between the N-type and P-type semiconductor material. Of concern in the choice of materials is the electrical conductivity and thermal conductivity of the thermoelectric material. A good thermoelectric material should have high electrical conductivity and low thermal conductivity. In this regard, additional low thermal k materials exhibiting improved electrical conductivity properties have been found for use as thermoelectric materials, of which included is boron carbide.
Traditionally, the Seebeck effect is defined as the ability to convert a temperature gradient from thermal energy into electrical voltage. By tapping into this voltage, electrical energy can be provided by a thermoelectric device, or module. Of concern in the generation of thermoelectric power is the ratio of the electrical voltage over the temperature gradient in relationship with electrical conductivity and thermal conductivity of a thermoelectric material. A good thermoelectric material should provide higher voltage with a given temperature gradient that is supported by a given heat flux through the material.
Semiconductor materials that are typically utilized as thermoelectric materials have a narrow energy forbidden gap, and provide for the free movement of electrons and holes in the conduction and valance band of the material. As a result, the temperature gradient resulting from a given heat flux through the material does not provide for optimal performance due to a high thermal conductivity of the free electrons and holes. Hopping electron conductive materials have been utilized in which it has been found that hopping electron conductivity reduces the heat carried by the electrons where the reduced electrical conductivity per carrier is offset by increasing the number of hopping carriers and hopping sites, thus providing for the desired low thermal conductivity. Reducing the thermal conductivity of the electrons is accomplished by reducing the mobility of the carrier by forming narrower bands within the energy forbidden gap or by introducing localized traps for the electrons. This hopping electron characteristic found in these materials provides for an improved figure of merit Z=S2Oxe2x80x2e/k, where S represents the Seebeck coefficient, Oxe2x80x2e represents the electron conductivity, k represents the thermal conductivity, including both the lattice and electronic components of the thermal conductivity. Hopping electrical conductivity has additionally been found to enhance the charge redistribution effect, thus enhancing the Seebeck voltage of hopping electron materials over simple semiconductors. It is understood that hopping electrical or conductive materials can be both n-and p-type.
The current method of fabricating a hopping electron conductor, or thermoelectric material, is to deposit a two-dimensional or three-dimensional quantum well boron carbide structure on to a silicon substrate using state-of-the-art processing capabilities, such as MBE. During fabrication, the materials are heated to a high degree, thereby enhancing crystallinity. The end result is a thermoelectric material having good electrical conductivity, but with remaining unwanted thermal conductivity of the substrate. This process however is impractical in practice due to the use of MBE processing in the fabrication of this thermoelectric material, which is difficult to use in the manufacture of large modules.
With respect to thermoelectric materials, boron carbide has been shown to exhibit modest thermoelectric performance, however fully carbonated perfect boron carbide has a forbidden energy gap of about 3 eV and hence serves as an insulator instead of a good conductor. The modest thermoelectric performance of boron carbide is therefore related to the defects in the material. In addition, boron rich boron carbide has a high Seebeck coefficient. There are however, a limited number of these defects that can be induced in simple boron carbide under manufacturing processing conditions. By varying these manufacturing process conditions, thermoelectric performance is achieved.
Accordingly, to overcome these problems, it is a purpose of the present invention to provide for a means for increasing the Seebeck voltage of a fully carbonated boron carbide material.
It is yet a still further purpose of the present invention to provide for a thermoelectric material having increased stability.
It is yet a still further purpose of the present invention to provide for a boron carbide based material with enhanced thermoelectric performance.
It is still a further purpose of the present invention to provide for a thermoelectric material having a more energetically favorable carbon chain and method of fabricating the thermoelectric material. The advantages are easy process control, better material stability, and increased thermoelectric properties.
It is a still further purpose of the present invention to provide a high performance thermoelectric material that does not require deposition of quantum wells.
The above problems and others are substantially solved and the above purposes and others are realized in a thermoelectric material comprising a Group IV element boride, such as carbon boride, or silicon boride, which is doped with a doping element chosen from the group consisting of Group III, IV or V elements, wherein the doping element is different from the Group IV element in the Group IV element boride, and the doping element is not boron. In addition, disclosed is a method of fabricating a thermoelectric material including the steps of providing Group IV element boride, such as carbon boride, or silicon boride, and doping the Group IV element boride with a doping element chosen from the group consisting of Group III, IV or V elements, wherein the doping element is different from the Group IV element in the Group IV element boride, and the doping element is not boron. An alternate method of fabricating a thermoelectric material is also disclosed including the steps of simultaneously growing to a desired concentration level on a substrate a Group IV element boride and at least one doping element chosen from the Group III, IV or V elements, wherein the doping element is different from the Group IV element in the Group IV element boride, and the doping element is not boron.